Upcycling fish scales through heating for steganography and Rhodamine B adsorption application

With increasing population and limited resources, a potential route for improving sustainability is increased reuse of waste materials. By re-looking at wastes, interesting properties and multifunctionalities can be discovered in materials previously explored. Despite years of research on bio-compatible fish scales, there is limited study on the fluorescence property of this abundant waste material. Controlled denaturation of collagen and introduction of defects can serve as a means to transform the fluorescence property of these fish scale wastes while providing more adsorption sites for pollutant removal, turning multifunctional fish scales into a natural steganographic material for transmitting text and images at both the macroscopic and microscopic levels and effectively removing Rhodamine B pollutants (91 % removal) within a short contact time (10 minutes). Our work offers a glimpse into the realm of engineering defects-induced fluorescence in natural material with potential as bio-compatible fluorescence probes while encouraging multidimensional applicability to be established in otherwise overlooked waste resources.

Tail (Supplementary Fig. 3).However, the fluorescence enhancement lacks uniformity, which can be resolved by optimizing the heating temperature and duration for different fish scale types.Generally, a piece of the fish scale (top view, Supplementary Fig. 4a) comprises different regions -the Anterior, the Lateral, the Focus, and the Posterior.Through cross-section imaging, the top layer (in contact with the fish's living environment) is the limiting layer (LL).Below the LL is the elasmodine, which comprises discrete piles of unidirectionally aligned collagen fibers.This elasmodine is further divided into two layers, the external elasmodine (EE) and the internal elasmodine (IE).The subtle distinction between these two layers lies in the latter having lower mineral content (Supplementary Fig. 4b).TG curve of the pristine primitive fish scale (Supplementary Fig. 6) shows predominately the release of adsorbed H2O, physisorbed CO2, and degradation of collagen and HAp, which agrees with TG curves from the separate entities of collagen and HAp (Fig. 2b-c).
Comparison of the FWHM of the XRD peaks from pristine and heated Hap extracts.The global PL of the heated fish scale is fitted to eleven Gaussian curves (Supplementary Fig. 10d).The fitted results suggest that the observed fluorescence comprises two main peaks at cyan and infra-red ranges, at (547.2 ± 0.6) nm and (890.1 ± 0.2) nm, respectively, and nine smaller peaks located at (390.5 ± 0.6) nm, (416.3 ± 0.8) nm, (447.6 ± 0.7) nm, (462.8 ± 0.1) nm, (486.9 ± 0.3) nm, (521.6 ± 0.7) nm, (780.7 ± 0.4) nm, (829.8 ± 0.2) nm and (966.5 ± 0.1) nm.The main contributors to the individual deconvoluted peaks in the heated fish scale are determined by comparing the deconvoluted graphs from both heated collagen (Supplementary Fig. 10e) and heated HAp (Supplementary Fig. 10f) extracts and indicated in Supplementary Supplementary Fig. 1.Systematic temperature study of heat-induced fluorescence enhancement in fish scale.The exposure time of this image is intentionally reduced to show the loss of fluorescence enhancement due to charring (enclosed in the yellow circle) when heated at 280 o C.
on the effect of heating duration at a constant temperature of 270 o C on the enhancement of fluorescence in fish scale.Supplementary Fig. 2. A systematic study on the effect of heating duration at a constant temperature of 270 o C on fluorescence enhancement in fish scale.Enhancement of the fish scale's fluorescence without charring the fish scale was determined to be the most intense for a heating duration of 3 minutes.At 4 minutes, charring of the fish scale resulted in a slight reduction in the fluorescence intensity, like Supplementary Fig. 1.Red Tilapia, b Yellow Tail and c Salmon scales.Enhanced fluorescence is also observed on heated fish scales (270 o C, 3 minutes) from Salmon and Yellow interest of a single primitive fish scale.Supplementary Fig. 4. a Top and b Cross-section of the fish scale.
pristine and heated NaCl to prove that NaCl does not contribute to the fluorescence observed in heated HAp and collagen extracts.Supplementary Fig. 5. FM images of pristine and heated NaCl observed under BF and UV excitation.The possibility of a fluorescence contribution from NaCl formed during extraction is eliminated with the lack of fluorescence observed from pristine and heated NaCl samples.The loss of mass, onset and end temperature corresponding to changes to the collagen and HAp structures are as indicated.

Table 1 .
The table includes the location of the diffraction peaks, lattice planes and the FWHM of each peak from pristine and heated HAp extracts.